1. Field of the Invention
The present invention is related to a method of manufacturing a Group III-V compound semiconductor by thermal decomposition vapor phase method using metalorganics.
2. Description of the Background Art
Since Group III-V compound semiconductors which are expressed by a general formula InxGayAlzN (0xe2x89xa7xxe2x89xa71, 0xe2x89xa7zxe2x89xa71, x+y+z=1) have a band gap which can be controlled by the composition of Group III elements, Group III-V compound semiconductors can be used as a light emitting element which emits light ranging from the visible range to the ultraviolet range. In addition, since Group III-V compound semiconductors have a band structure of the direct transition type, using a Group III-V compound semiconductor, it is possible to obtain a light emitting element which has a high luminescence efficiency. Group III-V compound semiconductors in which the concentration of In is 10% or more, in particular, are important for display applications, since the violet range or the longer visible wavelength range can be used as an emission wavelength.
A popular method of manufacturing Group III-V compound semiconductors is molecular beam epitaxy (hereinafter abbreviated as xe2x80x9cMBExe2x80x9d in some cases), metalorganic vapor phase epitaxy (hereinafter abbreviated as xe2x80x9cMOVPExe2x80x9d in some cases), etc. The MOVPE method is a method in which material gas is sprayed onto a heated substrate together with carrier gas so as to grow crystal due to thermal decomposition of the materials. Allowing crystals to grow uniformly over a wide area at a high accuracy, this method is industrially important as a manufacturing method. In the MOVPE method, hydrogen is often used as carrier gas since hydrogen is cheap gas with a high purity. This is because very pure hydrogen is relatively easily obtained by passing hydrogen through a palladium film.
Further, to efficiently inject a current into a semiconductor light emitting element with a low applied voltage, a p-type semiconductor and an n-type semiconductor are generally used as a current injection layer for holes and a current injection layer for electrons, respectively. However, in such a compound semiconductor manufactured by MOVPE, it is known that it is difficult to control the p-type conductivity while it is relatively easy to control the n-type conductivity. That is, even if doped with p-type impurities, the compound semiconductor has a high resistance. In general, therefore, the compound semiconductor is processed after grown, by electron beam irradiation, thermal annealing, etc., so that the compound semiconductor of the p-type conductivity has a low resistance.
Hence, while it is essential to perform such post-processing to manufacture an element which has a high current injection characteristic, such post-processing unavoidably deteriorates the yield. Moreover, a layer which contains the p-type impurities needs be located at the top and the layer must not be thick to render the post-processing effective, which serves as a restraint to the structure of the element.
Japanese Patent KOKAI (Laid-open) No. 6-232451 discloses that it is possible to grow p-type GaN, without performing any special post-processing, if grown by a method in which material gas is pressed against a substrate by second carrier gas which is blown from a direction which is perpendicular to a direction of supplying the material gas (hereinafter abbreviated as xe2x80x9cTFMOCVDxe2x80x9d in some cases). Still, this method aims at growing Mg-doped GaN on InGaN which was already grown, and therefore, the Mg-doped GaN does not exhibit the p-type conductivity if an InGaN layer is not used, according to the gazette.
Accordingly, an object of the present invention is to provide for a method of manufacturing a Group III-V compound semiconductor, which grows a nitrogen-contained Group III-V compound semiconductor of the p-type conductivity, without performing any particular post-processing after growing the compound semiconductor, and which prevents a deterioration in the yield of manufacturing light emitting elements due to post-processing.
Having studied the problems above and found that when inert gas other than hydrogen is mainly used as carrier gas, a compound semiconductor which contains p-type impurities exhibits the p-type conductivity even if the semiconductor is not processed by any particular post-processing, the inventors have arrived at the present invention. In other words, the present invention is related to the following:
[1] A method of manufacturing a Group III-V compound semiconductor which contains p-type impurities and which is expressed by a general formula InxGayAlzN (0xe2x89xa7xxe2x89xa71,0 xe2x89xa7yxe2x89xa71,0xe2x89xa7zxe2x89xa71,x+y+z=1), by thermal decomposition vapor phase method using metalorganics, the method being characterized in that carrier gas is inert gas in which the concentration of hydrogen is 0.5% or smaller by volume.
[2] A method of manufacturing a Group III-V compound semiconductor which contains p-type impurities and which is expressed by a general formula InxGayAlzN (0xe2x89xa7xxe2x89xa71,0xe2x89xa7yxe2x89xa71,0xe2x89xa7zxe2x89xa71, x+y+z=1), by thermal decomposition vapor phase method using metalorganics, the method being characterized in that after etching within a reaction furnace using at least one compound which is selected from a compound group consisting of compounds of halogenated hydrogen, compounds of halogen and Group V elements, and compounds of halogen, hydrogen and Group V elements, inert gas in which the concentration of hydrogen is 0.5% by volume is used as carrier gas.
[3] The method of manufacturing a Group III-V compound semiconductor as described in the paragraph [1] or [2], characterized in that when there are two or more inlets for introducing carrier gas and/or material gas, the maximum angle between any two of the inlets and any point on the substrate is 80 degrees or smaller.
[4] The method of manufacturing a Group III-V compound semiconductor as described in the paragraph [1],[2] or [3], characterized in that the concentration of hydrogen is maintained at 0.5% or smaller by volume in the atmosphere, during a reduction in the temperature of a Group III-V compound semiconductor after the Group III-V compound semiconductor is grown.
Now, the present invention will be described in detail.
In the present invention, the gas contains 0.5% of or less hydrogen by volume. If the carrier gas contains more than 0.5% of hydrogen by volume, the p-type conductivity is not good enough, which is not desirable. Reducing the concentration of hydrogen in the carrier gas, the larger p-type carrier concentration and hence the better characteristics as a p-type semiconductor are achieved. A preferable concentration of hydrogen is 0.3% or less, more preferably, 0.1% or less, and most preferably, 0.04% or less.
Further, even if carrier gas which does not contain hydrogen is used while growing a semiconductor, the semiconductor may not exhibit the p-type conductivity in some cases if the atmosphere contains hydrogen while the semiconductor is cooled down. Hence, the concentration of hydrogen in the atmosphere during cooling down is preferably low. Reducing the concentration of hydrogen in the atmosphere, the larger p-type carrier concentration and hence the better characteristics as a p-type semiconductor are achieved. A preferable concentration of hydrogen is 0.3% or less, more preferably, 0.1% or less, and most preferably, 0.04% or less.
In general, a compound semiconductor grows at a high temperature such as 800xc2x0 C. or more, and therefore, it is desirable to add Group V materials such as ammonia into the carrier gas while the temperature is not sufficiently lowered yet after growing the semiconductor, so as to suppress decomposition of the semiconductor due to heat. For this purpose, a preferable concentration range of Group V material in the growth atmosphere is 0.1% to 95%.
The inert gas, which mainly forms the carrier gas, is preferably helium gas, argon gas, nitrogen gas or a mixture of these, taking into consideration the chemical stability. Nitrogen is particularly preferable since highly pure nitrogen is relatively easily obtainable.
If the carrier gas according to the present invention is used from the beginning of the growth of crystal, the quality of the crystal may deteriorate, with a surface of the crystal not completed as a mirror surface or other problem. To deal with this, hydrogen gas may be used as carrier gas, first, to grow a compound semiconductor which has a high crystal perfection, and the carrier gas according to the present invention may be then used to grow a p-type layer, so that a layer with a high crystal perfection is formed.
Regarding the reaction furnace according to the present invention, while there is no problem when there is only one common inlet in the reaction furnace for introducing the carrier gas and the material gas (e.g., the example shown in FIG. 1), when there are two or more inlets for introducing the carrier gas and/or the material gas, it is preferable that the maximum angle between any two of the inlets and an optional point on a substrate is 80 degrees or smaller. More preferably, the maximum angle is 70 degrees or smaller. For instance, in FIG. 2, of five inlets for introducing the carrier gas and the material gas, angles between any two of the inlets and an optional point on a substrate may be xcex2 and xcex1. In this case, xcex2 less than xcex1. The maximum one of the angles between any two of the inlets and the optional point on the substrate corresponds to xcex1, and xcex1is preferably 80 degrees or smaller.
The angle between the direction of the inlet for introducing the carrier gas and the substrate surface is preferably 3 degrees or larger, or more preferably, 10 degrees or larger.
While the direction in which the material gas is supplied within the reaction furnace may be a direction from above to below, a horizontal direction, etc., in general, the direction of supplying may be a slanted direction or a direction from below to above.
When mixed with each other, Group V materials and Group III materials may produce an adduct with a low vapor pressure, resulting in a deterioration in the crystal perfection such as a deterioration in the surface morphology. To deal with such a case, Group V materials and Group III materials may be introduced through different pipes immediately until they are supplied onto the substrate and mixed with each other immediately before supplied onto the substrate, so that creation of an adduct is suppressed.
Further, in the present invention, it is desirable to etch the reaction furnace and the substrate by vapor phase etching before growing crystal, since such allows to grow high quality crystal with a good reproducibility. Etching gas may be a compound of halogenated hydrogen, a compound of halogen and Group V elements, or a compound of halogen, hydrogen and Group V elements. These materials may be used as they singularly are, or as they are mixed with each other. Of these materials, halogenated hydrogen, in particular, is preferable since halogenated hydrogen can be easily used. Hydrogen chloride is particularly preferable. If the etching gas is not very reactive with the substrate, the substrate may be etched when the reaction furnace is etched. Since this makes it possible to grow the semiconductor subsequently to etching, the compound semiconductor is grown with a good reproducibility, and almost without deteriorating the productivity.
The following materials can be used in the present invention. That is, materials of Group III elements include: triakylgallium which is expressed by a general formula R1R2R3Ga (where R1, R2 and R3 denote lower alkyl groups) such as trimethylgallium [Ga(CH3)3, hereinafter abbreviated as xe2x80x9cTMGxe2x80x9d in some cases] and triethylgallium [Ga(C2H5)3, hereinafter abbreviated as xe2x80x9cTEGxe2x80x9d in some cases]; triethylakylaluminum which is expressed by a general formula R1R2R3Al (the definitions of the symbols R1, R2 and R3 are the same as above) such as trimethylaluminum [Al(CH3)3], triethylaluminum [Al(C2H5)3, hereinafter abbreviated as xe2x80x9cTEAxe2x80x9d, in some cases], trisobutylaluminum [Al(ixe2x88x92C4H9)3; trimethylaminealane [AlH3N(CH3)3]; triethylakylindium which is expressed by a general formula R1R2R3In (the definitions of the symbols R1, R2 and R3 are the same as above) such as trimethylindium [In(CH3)3, hereinafter abbreviated as xe2x80x9cTMIxe2x80x9d] and triethyindium [In(C2H5)3]. These materials are used as they singularly are, or as they are mixed with each other.
Materials of Group V elements include ammonia or hydrazine, alklhydrazine such as methylhydrazine, 1, 1-dimethylhydrazine, 1-, 2-dimethylhydrazine. These substances are used as they singularly are, or as they are mixed. Since alkylhydrazine may pollute crystal with carbon, it is preferable to use ammonia, hydrazine, or to use ammonia and hydrazine as they are mixed together.
A p-type dopant may be Be, Mg, Zn, Cd, Hg, etc. Of these, Mg is particularly suitable for its high activation rate.
Zn materials may be alkylzinc which is expressed by a general formula R1R2Zn [where R1 and R2 denote alkyl groups] such as dimethylzinc [(CH3)2Zn] and diethylzinc [(C2H5)2Zn].
Mg materials may be biscyclopentadienyl magnesium [(C5H5)2Mg, hereinafter abbreviated as xe2x80x9cCp2Mgxe2x80x9d in some cases], bismethylcyclopentadienyl magnesium [(CH3C5H4)2Mg], bisisopropylcyclopentadienyl magnesium [(i-C3H7C5H4)2Mg], etc.
Cd materials may be alkylcadmium which is expressed by a general formula R1R2Cd [where R1 and R2 denote alkyl groups] such as dimethylcadmium [(CH3)2Cd].
Be materials may be diethylberyllium [(C2H5)2Be], bismethylcyclopentadienyl beryllium [(CH3C5H4)2Be], etc.
Hg materials may be alkylmercury which is expressed by a general formula R1R2Hg [where R1 and R2 denote alkyl groups] such as dimethylmercury [(CH3)2Hg], diethylmercury [(C2H5)2Hg].
Pressure to be applied during growth is preferably low, so as to realize such a velocity of flow with which a flow in the vicinity of a susceptor is not disturbed. Since In tends to be less incorporated as the pressure during growth becomes small, the pressure during growth is preferably large to a certain extent. To be specific, the pressure is preferably in the range between 1 atmospheric pressure and 1 Torr, and more preferably, in the range between 1 atmospheric pressure and 10 Torr.
A substrate for growing crystal used in the present invention may be a sapphire substrate, an SiC substrate, an Si substrate, a GaAs substrate, a ZnO substrate, etc. A sapphire substrate is particularly preferable, since a sapphire substrate grows a compound semiconductor with a very high crystal perfection as known in the art, when the semiconductor is grown by a two-step growth method which requires to grow a buffer layer on the substrate, first.
Substantially, the present invention does not need any post-processing to realize the p-type conductivity, and therefore, an element is manufactured through less manufacturing steps in the present invention than in other techniques, whereby the yield is improved. Further, while other techniques require the layer which contains the p-type impurities to be formed on the top so as to improve the efficiency of processing for realizing the p-type conductivity, such processing is not necessary in the present invention. Hence, there is no problem in fabricating an element even if the layer which contains the p-type impurities is formed on the substrate side to the active layer and the n-type current injection layer, which serves to reduce a restraint to the element structure.
Moreover, according to the present invention, it is possible to grow the active layer and the n-type layers at a low temperature after growing the p-type layer which has a sufficiently high carrier concentration at a high temperature. Hence, it is possible to grow the entire p-type layer without deteriorating the active layer, which in turn improves characteristics of a resultant element.
The color of emitted light from a light emitting element according to the present invention can be adjusted by a composition of Group III elements which are contained in the active layer. It is also possible to adjust the emission color, by doping the active layer with impurities. As impurities which alter the emission color when doped, Group II elements such as Be, Cd, Mg, Zn are suitable. Owing to its high luminescence efficiency, Zn is further preferable.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.